The invention concerns a main coolant pump for nuclear reactors with a particularly electric, drive motor and a housing which contains the pump impeller. Such pumps drive the coolant of the nuclear reactor through the reactor pressure vessel with the reactor core, where nuclear heat is generated, and then through a cooling loop which is located outside the reactor pressure vessel and transmits the heat, for instance, for the generation of steam. In pressurized-water reactors, for which the invention is primarily of interest, the coolant is water which is under a pressure of, for instance, 150 bar.
If a leak occurs in such a nuclear reactor in a coolant line associated with the primary coolant pump, the coolant escapes with great velocity. Under unfavorable conditions, a flow can therefrom develop in the pump, for which the pump impeller acts as a turbine, so that the pump is accelerated by the escaping coolant. If the leak occurs in the cooling loop between the pump and the pressure vessel, the pump's impeller which normally circulates the coolant in that direction, can be accelerated in its normal rotative direction, to abnormal rotative speeds. If the leak occurs in the loop on the other side of the pump, counter to the pump's normal coolant circulating direction, the pump's impeller is first decelerated to a stop and then driven in a rotative direction counter to its normal rotative direction, and possibly to abnormal rotative speeds. This reversal and reverse driving of the impeller, can occur in a matter of seconds, because the pump's rotative mass is small compared to the volume and high pressure of the pressurized-water coolant in the pressure vessel and cooling loop. This can go so far in case of an extensive leak, that overspeed leads to jeopardizing the mechanical strength of the pump. A particular danger here is that not only damage to the pump itself is involved, but above all that pump parts that may be flung off by centrifugally-explosive speeds, can damage important components of the nuclear reactor. It is therefore an object of the invention to seek remedial action for the case described.